1. Field of the Invention
The present invention relates to a filtration method for water employing a membrane filter, and a filtration apparatus.
2. Description of the Prior Art
FIG. 3 is a perspective view showing the construction of a conventional filtration apparatus.
FIG. 4 is a schematic representation of a construction of different conventional filtration apparatus.
FIG. 3 and FIG. 4 show a filtration apparatus in which an ultramembrane filter, a micromembrane filter or a nonwoven fabric is disposed in a filtration tank into which air is blown. The growth of algae on the membrane surface results in a layer of material that makes membrane filtration impossible because of hydraulic resistance of the algae layer. This can occur within one to two weeks after a new membrane is placed in service for recirculation of pool water and fish breeding water, for drinking water from river water and for filtration of sewage treated with activated sludge.
As a countermeasure for fouling of the membrane filter as described above, sponge balls are usually placed in the filtration tank to remove the algae layer on the membrane filter. The sponge balls contact the filtration surface of the membrane facilitating peeling of the algae off the surface. However, this measure has a small removal effect along with having a drawback that the sponge balls are drained together with the sludge accumulated in the filtration tank, requiring a cost for supplementing the sponge balls. Otherwise, sodium hypochlorite is added into the filtration tank to prevent growth of algae or to disinfect algae. The effective chlorine concentration, then, should be maintained at a certain level, for example at 0.5 ppm, in this method. Chlorine residuals are toxic to aquatic life in receiving waters and the by-products of chlorinating may be carcinogenic. Moreover, growth of microorganisms for decomposing inactivated algae in the activated sludge is inhibited with added chlorine, thereby colloidal organic substances produced from incomplete decomposing of inactivated algae may foul of the membrane filter.